Heating and Current Drive Systemsfor ARIES-AT

T.K. MauUniversity of California, San Diego

ARIES Project MeetingSeptember 18-20, 2000

Princeton Plasma Physics LaboratoryPrinceton, NJ

OUTLINE

• CD Analysis for ARIES-AT equilibria at = 9.1% (90% of limit) and with R = 5.2 m, Ip ~ 13 MA, and Bo = 5.9 T

• Power requirement, profile alignment and number of CD systems

• Normalized CD efficiency scaling vs Te and Zeff.

• RFCD launcher system definitions

• Conclusions and Discussions

Seed CD Requirements for Latest ARIES-AT Equilibria

• Latest series of ARIES-AT equilibria have profiles optimized to give high N ( 90% of limit ), and maximum bootstrap alignment ( Ibs/Ip > 0.9 ) at Zeff = 1.7, Te0 = 24, 26, 28 and 30 keV.

• Seed current is defined as: Jsd = jeq - jbs - jdia - jps in - direction.

• Bootstrap alignment: 2 regions of seed CD : (1) On axis; (2) Off axis

• CD power and system requirements determined by driving seed current profile using RF techniques.

n, T profilesRS core, L-mode edge

Te

ne Te0 = 26 keVN = 5.4fbs = 0.917

EQBS

Dia+PSOn-axisSeed: 0.04 MA

Off-axisSeed:1.05 MA

Seed CD Requirements at Zeff = 1.8

Modified n, T profilesRS core, L-mode edge

Te

neTe0 = 26 keVN = 5.4fbs = 0.897

BS

EQ

On-axisSeed: 0.03 MA

edge+mid-radiusSeed: 1.32 MA

Dia

• Bootstrap current is sensitive to changes in Zeff.

• To extrapolate from Zeff = 1.7, adjust n and T profiles to obtain bootstrap alignment without overdrive.

• Three regions of seed current: (1) on-axis seed : < 0.2, (2) mid-radius seed : 0.5 < < 0.8 (3) edge seed : > 0.8.

Current Drive at Zeff = 1.7

• Needs two CD systems:

1. ICRF/FW for on-axis drive : < 0.2; Pfw ~ 1-2 MW 2. LHW for off-axis drive : > 0.8; Plh ~ 25-40 MW

• Very good current alignment can be obtained.

Teo = 26 keVfbs = 0.917Pfw = 1.4 MWPlh = 32 MW

FW

LH

BS

EQRF

Dia

Teo = 30 keVfbs = 0.911Pfw = 2.2 MWPlh = 36 MW

EQ

BS

RF

FW Dia

LH

Current Drive at Zeff = 1.8

• Three CD systems are required:

1. ICRF/FW for on-axis drive : < 0.2; Pfw ~ 1 MW 2. LHW for off-axis drive : > 0.8; Plh ~ 30-40 MW 3. HHFW for mid-radius drive : 0.5 < < 0.8 ; Phh ~ 10-16 MW

• Fair current profile alignment

Teo = 24 keVfbs = 0.897Pfw = 1.1 MWPlh = 40 MWPhh = 16 MW

Teo = 28 keVfbs = 0.898Pfw = 0.8 MWPlh = 32 MWPhh = 16 MW

FW Dia HH

LH

BS

EQ RF

FW Dia HH

LH

BS

EQ RF

2.5

3

3.5

4

4.5

23 24 25 26 27 28 29 30 31

Zeff=1.7

1.6

1.8

,Peak Electron Temperature T0e( )keV

-ARIES AT=4, =5.2A R m

=9%

CD Efficiency Scaling vs Te0 and Zeff

B = <n>IpRo/PCD

• Based on four equilibria optimized at Zeff = 1.7 and Te0 = 24, 26, 28, 30 keV. Thus, Zeff = 1.7 case has the highest CD efficiency.• For Zeff = 1.7 and 1.6, only 2 RF systems are required (FW+LH).• For Zeff = 1.8, 3 RF systems are required (ICRF/FW+LH+HHFW). Alignment not as good: results are less reliable.

T

D

2T

2D,3T

4T3D

5T 4D,6T

96 MHz

68 MHz

135 MHz

22 MHz

Frequency Options for Fast Wave On-Axis CD

• Criteria : Avoid ion and absorption no resonance on OB side Reasonable antenna size higher frequency

• 68 MHz, 96 MHz, and 135 MHz appear feasible; similar power requirements• 68 MHz is used in most calculations.

R-a R+a

Axis

ICRF Fast Wave Drives On-axis Seed Current

• Wave frequency is chosen to place 4fcT resonance at R > Ro+a, and 2fcD resonance at R << Raxis, to minimize ion and alpha absorption.

• Launcher is located on outboard midplane with N|| = 2 spectrum for best current profile alignment.

• Plasma & wave parameters : R = 5. 2 m, A = 4, = 2.2, =0.8, Bo = 5.9 T, Ip = 13 MA, N = 5.4, Teo = 26.8 keV, neo,20 = 2.83, Zeff = 1.8 f = 96 MHz, N|| = -1.5.

Z (

m)

R (m)X (m)

Y (

m)

Axis

ARIES-ATPe/P = 0.90PT/P = 0.02P/P = 0.08I / P = 0.036 A/W

Dri

ven

Cu

rren

t

Off-Axis/Edge Seed CD with LH Waves

• Frequency = 3.6 GHz [ > 2 * fLH (=0.8) ] - Less than 1% alpha absorption• Usually five waveguide modules, each launching a different N||, are required. - Located ~2 m. below OB midplane to give maximum penetration.• Penetration to < 0.8 is not possible for this class of AT equilibria. Low N|| rays encounter mode conversion to fast wave at >0.8 and propagates back to edge; higher N|| rays get totally damped before reaching = 0.8.

start

end

N|| = -1.6

e-dampinglimit

MC limit

InaccessibleAccessible

Mid-Radius CD Using High Harmonic Fast Waves (HHFW)

• At f ~ 20fci, HHFW can penetrate deeper than LH waves.

• CD efficiency is found to be acceptable.

• Issues: - Strong absorption by energetic ’s - Experimental database being developed on NSTX at 30 MHz. - No credible FW launcher design at f ~ 0.9 GHz.

F = 0.9 GHzN|| = -2

Te0 = 26 keVZeff = 1.8P/P = 0.41

AbsorptionCurrent Drive

Te0 = 26 keVZeff = 1.8I/P = 0.018 A/W

e

Current Drive System Definition for ARIES-AT

• Reference Option : ICRF/FW + LHW

- Requires two RF systems and highly compatible with core configuration - Requires lowest CD power (30-40 MW) - Likely narrow range of operation - Issues : (1) LH wave penetration limited to > 0.8. • Second Option : ICRF/FW + HHFW + LHW

- Requires three RF systems; should be compatible with core design - Requires more CD power (40-60 MW) - Broader range of operation - Issues: (1) alpha absorption of HHFW power (2) HHFW antenna concept remains to be developed.

• Comments:

- Because of small on-axis seed current, ECCD can be a viable alternative to ICRF/FW. - Should extra ICRF power be set aside for auxiliary heating? Can existing CD systems heat plasma to design point?

Definition of the ICRF Fast Wave Launcher System

• Assumed requirements for Zeff = 1.8, Te0 = 26 keV (strawman): - 1 MW of power @ 96 MHz and N|| = 1.5 for on-axis CD. At 96 MHz, similar jfw profile and I/P are obtained. Higher frequency is used to reduce size of launcher.

• Base launcher module is similar to ARIES-RS folded waveguide design : - Has 8 waveguides in a toroidal array, with 45o phase shift - Each waveguide has 10 folds - Located at outboard midplane - Radial thickness with diaphragm = 0.97 m - Module dimensions are : 2.08 m (width) x 0.51m (height) with total aperture area = 0.99 m2

• Taking a maximum power density of ~40 MW/m2, prudence requires us to set the power limit at ~20 MW. Extra power can be used for auxiliary heating and/or rotation drive. • Structural material is SiC with W coating (as in divertors); high surface resistive dissipation [TBD]; structures (Faraday shields, straps and support) to be cooled with LiPb. Other choices will be explored.

Isometric View of Folded Waveguide Unit

• Design and dimensions are similar to ARIES-RS (f = 95 MHz)

Definition of the LH Wave Launcher System

• Calculated lower hybrid system requirements for Zeff = 1.8, Te0 = 26 keV: - 5 waveguide modules delivering a total power of 35 MW. Module frequency (GHz) N|| Power (MW)

1 3.6 1.7 1.1 2 3.6 2.0 5.9 3 3.6 2.5 7.0 4 3.6 3.5 7.5 5 2.5 5.0 13.9

• Base unit is the passive/active multijunction grille, modeled after ITER-EDA design, and used in ARIES-RS.

• The grilles are located at ~2 m from the outboard midplane.

• Using ITER guideline for power flux capability: P (MW/m2) < 20 f 2/3(GHz), total required port area = 1.34 m2.

R

φ

θ

67.8cm69.8cm

22.5cm

mouthpiece

hyperguide

emitterpiece

N||=2.0

35.0cm

N||=1.8

* 4.6 4 ,2 The GHz LH launcher array consists of units for N||=2.0 2spectrum and forN||=1.8 .spectrum

* =20 ; =0.7.Total power transmitted MW Directivity

24cm

Front View of LH Launcher Modules

• Shown are the designs for ARIES-RS, for illustration purpose only.

Consideration of HHFW Launcher System

• Calculated HHFW system requirements for Zeff = 1.8, Te0 = 26 keV:

- Launched wave spectrum at 0.9 GHz and N|| = 2.0. - Launch location : outboard midplane. - Power = 16 MW.

• At present, there is no proven design of FW launcher in 0.9 GHz range. Possibilities include:

- Combline structure : data at 200 MHz (GA/JFT-2M) - Folded waveguide : no data close to 0.9 GHz

• Assume similar power scaling as ITER guideline for LH waves:

- At 0.9 GHz, power density limit = 18.6 MW/m2 (conservative!) - First wall penetration area = 1.16 m2.

ICRF/FW

LHW LHW

LHW LHW

LHW

HHFW

BlanketSector

Special Blanket Sector with RF Launchers

• There are 16 blanket sectors. One sector has a width of ~2.6 m at midplane.

• Sketch of locations of RF launchers in the sector is based on Zeff = 1.8, Te0 = 26 keV (strawman).

• Aperture area for the launchers:

- ICRF/FW : 0.99 m2

- LHW : 1.34 m2

- HHFW : 1.16 m2

Total aperture area = 3.49 m2

= 1% of first-wall area.

Conclusions and Discussions

• A series of ARIES-AT equilibria with N = 5.4 and fBS = 0.91 at Zeff = 1.7 and Te0 = 24, 26, 28 and 30 keV have been analyzed for CD power and launch requirements. Extrapolations to Zeff = 1.6 and 1.8 are made. • CD efficiency scalings were calculated vs Te0 and Zeff; 2 RF systems are required for Zeff = 1.6, 1.7, while 3 systems are required for Zeff = 1.8, resulting in lower fBS and higher CD power requirements.

• Based on the present strawman with Zeff=1.8 and Te0 = 26 keV, 3 RF systems are required: LHW for edge CD, ICRF/FW for on-axis CD and HHFW for mid-radius CD. Power requirement is reasonable at ~ 52 MW level. Extra ICRF power for auxiliary heating and/or rotation drive should be provided. • Launcher designs for both LH and ICRF systems have been on-going.

• Initial design results in launcher penetration equal to 1% of first wall area. It appears feasible to place all RF modules in one blanket sector.

Suggested Remaining Tasks

• CD power may be lowered, and number of RF systems may be reduced to two by looking at equilibria optimized at Zeff = 1.8 or higher, and with no mid-radius seed current drive ( 0.5 < < 0.8 ).

• Complete detailed design of ICRF/FW and LHW launchers.

- Dimensions of various modules - Wall dissipation with W coating on structures, and compare to Cu.

• Address the issue of auxiliary heating during start-up with existing CD systems: - How much extra ICRF power is required? At what frequency? - What are the implications for using LHW to heat the plasma?

Issues and Areas for Future Research

• Heating and Current Drive:

- LHW penetration is limited in high- plasma; HHFW is a possibility, but needs innovative antenna concept; - Investigate the dynamics of RF current profile control --- modeling, and physics and technological constraints - Refine modeling capability to self-consistently determine MHD stable equilibrium with bootstrap and externally driven currents; - Use wave spectrum calculated for RF launcher in ray tracing analysis; - Study roles of RF in rotation generation and transport barrier control

• RF Launcher:

- EM field analysis inside folded waveguide in realistic geometry, and experiments in a tokamak environment - Detailed launcher cooling and thermal stress analysis - Structural material choice in SiC environment : SiC with metal coating - Wave coupling and loading during plasma transients